1. Field of the Invention:
The present invention relates to a hydraulic shock absorber for use, for example, in a vehicle, wherein the level of the damping force is adjustable in accordance with the road surface conditions or the like during the operation of the vehicle.
2. Description of the Prior Art:
There have heretofore been proposed various kinds of hydraulic shock absorbers for use for example, in automobiles which are designed so that the level of the damping force can be properly adjusted so as to be suitable for the road surface conditions during the operation of the vehicle in order to improve the driveability and enable the driver to enjoy a comfortable ride.
One type of conventional damping force adjustable hydraulic shock absorber is arranged such that a bypass passage which provides communication between two cylinder chambers, defined in a cylinder by a piston, is formed in a piston rod and a plurality of orifices are provided in the intermediate portion of the bypass passage so as to be selectively opened and closed by a shutter which is externally operated, thereby enabling a change in the level of damping force generated by means of the orifices, that is, a change in the orifice characteristics see [for example, Japanese Utility Model Public Disclosure (Kokai) No. 61-1729 (1986)].
Another type of conventional damping force adjustable hydraulic shock absorber is arranged such that a first damping force generating valve is provided in the piston, while a second damping force generating valve, which has a relatively low fluid resistance, is provided in the intermediate portion of a bypass passage providing communication between two cylinder chambers defined in the cylinder, so that it is possible to change the level of damping force generated by the second damping force generating valve, that is, the valve characteristics, by selectively allowing working fluid to flow to the second damping force generating valve, or cutting off the flow of the working fluid by opening or closing the bypass passage by means of an externally operated shutter [see, for example, Japanese Utility Model Public Disclosure (Kokai) No. 59-11932 (1984)].
The above-described conventional damping force adjustable hydraulic shock absorbers suffer however, from the following problems.
First, in the prior art arrangement designed so that the level of damping force generated by means of orifices is variable, it is possible to obtain various characteristics of damping force generated by means of the orifices when the movement of the piston is within the low-speed region by properly selecting the orifices, but the damping force characteristics in the high piston speed region are principally determined by the characteristics of the damping force generating valve and therefore cannot be set as desired; that is, there is little room for choice. In addition, the range within which the level of the damping force can be varied is narrow. Therefore, it is impossible to obtain a level of damping force which is satisfactorily adequate for the road surface conditions when the movement of the piston is in the high-speed region.
In the prior art arrangement wherein a plurality of damping force generating valves are properly switched over, it is possible to set an optimal level of damping force in conformity with the road surface conditions, but it is necessary to provide a plurality of damping force generating valves for each of the extension and contraction strokes of the hydraulic shock absorber which increases the size and complicates the structure of the shock absorber, resulting in an increase in the weight of the product and a rise in its costs. In addition, maintenance of this arrangement is difficult.